1. Field of the Invention
The present invention relates to a liquid-discharge recording apparatus and, more particularly, to a liquid-discharge recording apparatus having the mechanism to maintain a viscosity of an ink to be emitted so as to be fitted for emission at least when the ink is emitted.
2. Description of the Prior Art
According to liquid-discharge recording apparatuses, a recording liquid (for example, ink) is held in an ink vessel, the ink is led to a recording head unit from this ink vessel, a nozzle provided in the recording head unit is driven in response to a print pattern signal, and at the same time the ink is emitted from a discharge opening at the head of the nozzle, thereby performing the recording such as the printing or the like on a recording material such as a paper or the like. The ink emitted forms a jet liquid droplet and is deposited on the recording material.
As methods of emitting the ink onto the recording material, the method whereby an electromechanical converter such as, e.g., a piezoelectric device or the like is used, the method whereby an electrothermal energy converter is used, and the like are known. According to the method whereby the electrothermal energy converter is used, the ink in the nozzle is heated by the electrothermal energy converter to cause a change in pressure of the ink, thereby emitting the ink.
In the liquid-discharge recording apparatuses to which the above-mentioned emitting methods and other conventional emitting methods are applied, it is a general manner that the discharge opening at the head of the nozzle to emit the ink is always open into the open air irrespective of whether the apparatus is operating or not. Therefore, in the case where the recording is not performed for a long time, the water and volatile organic solvent or the like which are the components of the ink evaporate into the open air from the inks remaining at the discharge opening and in the portion near the discharge opening. Thus, the viscosity of the residual ink increases and exceeds a range of viscosity necessary for emission, causing a problem such that no ink is emitted in spite of the fact that a print signal is applied immediately after the apparatus operated and the recording was restarted.
In addition, there is also another problem such that a temperature of the ink decreases at low temperatures in winter season or the like, so that the viscosity of the ink also increases.
To solve the problem of the increase of the viscosity of the ink mentioned above, there has been proposed the method whereby the ink is heated just before the recording is restarted, namely, just before the ink is again emitted, and the temperature of the ink is increased, thereby reducing the viscosity and maintaining it to a predetermined viscosity range.
However, for such a preheating, the heating condition is largely changed due to the circumstances under which the recording apparatus is used. Namely, there is a drawback such that if the preheating condition is determined so as to obtain good emission of the ink droplets even under low temperature environment (for example, 5.degree. C.), the viscosity of the ink becomes too low due to the heating under high temperature environment (e.g., 35.degree. C.), so that the ink viscosity is out of the range necessary for good emission. On the contrary, in the case where the preheating condition is set so as to derive a good ink viscosity under high temperature environment, the necessary viscosity cannot be derived under low temperature environment.
Further, even when the environmental temperature is constant, the heating condition is also largely changed depending on the use state of the recording apparatus. Namely, due to the use of the recording apparatus, all of the thermal energy applied from the electrothermal energy converter to emit the ink droplets from the discharge opening, for example, is not necessarily used to form the ink droplets but a part of this thermal energy increases the temperature of the peripheral members of the electrothermal energy converter. Therefore, the temperature of the portion where the discharge opening is formed immediately after the completion of the recording is largely changed as compared with the temperature before the start of the recording, so that there is a problem such that, for instance, when the ink is heated at the restart of the recording just after the end of the recording, the ink is overheated and the viscosity overdecreases.
As another method for preventing the occurrence of the problem due to the increase of the viscosity of the ink, there has been proposed the method whereby the ink is always heated when the apparatus is used and the ink temperature is always kept constant in consideration of a change in temperature of the external environment and thereby to cope with the foregoing problem (Japanese Patent Unexamined Publication No. 187364/1983). As the ink heating means in this case, there are considered the method whereby the electrothermal energy converter which is used to form the flight ink droplets is used, and the method whereby another electrothermal energy converter is separately provided to always heat the ink.
However, the use of only the above methods also causes the problem such that it takes a time to heat the ink when the recording is restarted. Namely, in the case of using the electrothermal energy converter provided to form the ink droplets, it is possible to apply the electrical signal only within a range such as not to form any ink droplet. Therefore, an electrical signal of a high level cannot be applied and the heating time inevitably becomes long. In the case of using the electrothermal energy converter separately provided, such a limitation does not occur; however, since the heat is concentrated to only a single portion, the peripheral portion is influenced by the heat and the durability deteriorates. Thus, even in this case as well, an electrical signal of a fairly high level cannot be applied.
In addition to the foregoing heating methods, there has been further proposed the method whereby the recording head provided with the discharge opening in the recording apparatus is located at the home position when the recording is interrupted and this discharge opening is covered by a cap at the home position.
However, the use of both the heating of the ink and the capping of the discharge opening causes the viscosity of the ink to exceed the viscosity range necessary to emit the ink, so that the emission of the ink and the formation of the ink droplets are not always accurately performed. Further, in the case where the recording is interrupted for a long time, there is a risk such that the evaporation portion of the ink which is likely to be evaporated due to the heating of the ink leaks from the cap, so that the concentration of the ink near the discharge opening rather increases.